Otter Board Research Articles

Hydrodynamic characteristics of a hyper-lift otter board with wing-end plates

The paper investigates the hydrodynamic characteristics of a hyper-lift trawl door (HLTD), which consists of a cambered plate with two wing-end plates with the leading edge modified into an airfoil shape. Flume experiments were carried out in a circulating water tank using models having a wing area of 625 cm2 and an aspect ratio of 1.0 at camber ratios of 15, 20, and 25 %. To determine the effect of the wing-end plates, models with different size of wing-end plates were tested. Moreover, the effect of heel angle on the hydrodynamic characteristics of the HLTD was also investigated. We concluded that the size of the wing-end plate was significant for h/c = 0.10 (h width of the wing-end plate, c chord length of cambered plate), and the maximum lift coefficients of HLTDs with camber ratios of 15, 20 and 25 % were 2.19, 2.38, and 2.48, respectively, at an attack of angle of 38°. In addition, our findings showed that the effect of the heel angle was very small on the lift, drag and moment coefficients of a HLTD when it slants at heel angles from −20° to 20° during trawling.

Comparing three conventional penaeid-trawl otter boards and the new batwing design

Three experiments were conducted to compare the engineering and catching performances of a hydrodynamic otter board termed the ‘batwing’ (comprising a sled-and-sail assembly, configured to operate at 20° angle of attack – AOA and with minimal bottom contact) against three conventional designs (termed the ‘flat-rectangular’, ‘kilfoil’ and ‘cambered’ otter boards) with AOAs between ∼30 and 40°. Experiments involved paired penaeid trawls (7.35-m headlines). The first experiment compared the batwing otter boards against all other designs (using 41-mm mesh trawls). In experiment 2, the batwing was tested against the flat-rectangular design (with 32-mm mesh trawls). In experiment 3, the batwing and flat-rectangular otter boards were towed without trawls to facilitate estimates of their partitioned drag. Overall, compared to the conventional otter boards, the batwings had up to ∼86 and ∼18% less bottom contact and drag, respectively. Among the conventional otter boards, the trawls spread by the cambered design caught up to 13% more school prawns Metapenaeus macleayi attributed to their greater solid profile. No significant differences were detected among catches of fish in the trawls spread by the various otter boards. The results reaffirm that because otter boards contribute towards a large proportion of total system drag (estimated here at up to ∼56%), their appropriate configuration is essential to maximise the fuel efficiency of penaeid-trawl systems.

The 'W' prawn-trawl with emphasised drag-force transfer to its centre line to reduce overall system drag.

For prawn trawling systems, drag reduction is a high priority as the trawling process is energy intensive. Large benefits have occurred through the use of multiple-net rigs and thin twine in the netting. An additional positive effect of these successful twine-area reduction strategies is the reduced amount of otter board area required to spread the trawl systems, which leads to further drag reduction. The present work investigated the potential of redirecting the drag-strain within a prawn trawl away from the wings and the otter boards to the centre line of the trawl, where top and bottom tongues have been installed, with an aim to minimise the loading/size of the otter boards required to spread the trawl. In the system containing the new ‘W’ trawl, the drag redirected to the centre-line tongues is transferred forward through a connected sled and towing wires to the trawler. To establish the extent of drag redirection to the centre-line tongues and the relative drag benefits of the new trawl system, conventional and ‘W’ trawls of 3.65 m headline length were tested firstly over a range of spread ratios in the flume tank, and subsequently at optimum spread ratio in the field. The developed ‘W’ trawl effectively directed 64% of netting-drag off the wings and onto the centre tongues, which resulted in drag savings in the field of ∼20% for the associated ‘W’ trawl/otter-board/sled system compared to the traditional trawl/otter-board arrangement in a single trawl or twin rig configuration. Furthermore, based on previously published data, the new trawl when used in a twin rig system is expected to provide approximately 12% drag reduction compared to quad rig. The twin ‘W’ trawl system also has benefits over quad rig in that a reduced number of cod-end/By-catch Reduction Device units need to be installed and attended each tow.

저층 트롤에서 3차원 위치를 이용한 만곡형 전개판의 영각 추정

The angle of attack of a cambered otter board in a bottom trawl was estimated using a three-dimensional semi-analytic treatment of a towing cable (warp) system that was applied to the field experiments of a bottom trawl obtained by the Scanmar system. The equilibrium condition of the horizontal component and vertical component of forces was used to the three forces acting on the otter board in the horizontal plane. Those forces were the force on the warp at the bracket, hydrodynamic lift and drag forces on the otter board and the force on the hand rope attached just behind the otter board. Also the equilibrium of moment about the front edge of the otter board was used to find out the angle of attack of the cambered otter board. When the warp length was 120m and 180m long and the towing speed was between 1.23 and 1.90 m/s, the estimated angle of attack of the cambered otter board was ranged between and , respectively, though the maximum lift force was at the angle of attack . The angle of attack of the otter board was tended to increase weakly with the longer length of warp (180 m) at the same towing speed in the experiment.

Model test of hydrodynamic characteristics of two types of vertical cambered slotted otter boards

Model test of hydrodynamic characteristics of two types of vertical cambered slotted otter boards

The effect of mesh orientation on netting drag and its application to innovative prawn trawl design

Prawn fisheries around the world comprise fuel intensive enterprises currently stressed financially by rising diesel costs. An avenue for relieving the situation is to improve the energy efficiency of trawling by raising the productivity of fishing per litre of fuel consumed. This paper presents work to develop a new prawn trawl design that leads to reduced trawl system drag. The trawl has a ‘double-tongue’ format, which refers to extensions forward of the upper and lower panels to form two additional towing points for the trawl. For this design concept, named ‘W’ trawl, drag generated in the trawl is largely directed to the centreline tongues and transferred forward to the trawler through a connected sled and towing wire. The associated reduction of drag-transfer to the wings makes the trawl substantially easier to spread and results in smaller otter boards being required and subsequently reduced overall drag of the trawl system. The study determined the effect on frame-line tensions of implementing T0 (diamond) and T45 (square) mesh in the main body and side sections of trawl models of conventional and ‘W’ configuration, with the aim to establish an optimal combination of mesh orientation for the principle parts of the ‘W’ trawl. The objective was to achieve minimum netting drag and beneficial strain transfer within the trawl such that maximum trawling performance (catch per unit of fuel) might be obtained in the field. T45 mesh in the side sections of the trawl was found to exhibit a progressively lower drag compared to T0 mesh as the flow speed increased, but the extent of drag reduction was not of practical significance. The ‘W’ trawl showed a capacity of redirecting 59% of the total netting drag to the centre line tongues when T45 netting was implemented in the body section, and only 40% when T0 orientation was used. However, the introduction of bracing ropes (at E=0.71) along the upper and lower centrelines of the T0 version of the “W” trawl improved the drag transfer to the tongues from 40% to 50% of the total drag. Overall, the most practical and economic configuration of the model ‘W’ designs tested produced an estimated drag reduction of 8.3%±0.6%, compared to the conventional trawl. It is expected that drag saving benefits in practice will be more substantial as the tested trawl models were not completely representative of practical commercial gear in that they had minimum twine area to make the experiment most sensitive to the drag-effect of mesh orientation.

A simplified concept for recovering a UUV to a submarine

The use of unmanned underwater vehicles (UUVs) to enhance the military capabilities of submarines and to reduce their operational risk is being considered by navies around the world. However, a major difficulty with the operations of UUVs is their recovery back to the submarine at the end of the mission. Various schemes have been proposed by a number of organisations working in this field; however, they all have major drawbacks. A simple concept is proposed based on a warp from the submarine to a low aspect ratio wing, similar to an otter board used by fishing trawlers. This approach allows the submarine to recover the UUV by slowly overtaking it. This is done at a sufficiently large transverse distance between the vessels where hydrodynamic interaction between the two is minimal, with the submarine travelling at a speed at which it can be safely controlled.

Design of trawl otter boards using computational fluid dynamics

Computational fluid dynamics (CFD) analysis was conducted for a biplane-type otter board model to validate the use of CFD analysis in designing otter boards. In the base mode (aspect ratio 3.0, camber ratio 15%, gap-chord ratio 0.91, and stagger angle 30°), CFD analysis results show a maximum lift coefficient value of 1.65 for angle of attack 17.5°, which is in agreement with results of the flume tank experiment (CLmax=1.64 when α=20°) under the same conditions. In addition, the streamline and separation zones around the otter board were visualized and compared to the results from past tank experiment studies. CFD analysis results showed patterns similar to the experimental results, and the details of flow with a trailing vortex were confirmed. We concluded that CFD analysis is useful in the designing and development of otter boards.

Comparing and modifying penaeid beam and otter trawls to improve ecological efficiencies

In an attempt to improve the selectivity and engineering performances of generic penaeid trawls, three established and one novel spreading-mechanism configurations were assessed: otter boards attached (1) with and (2) without 3.15-m sweeps to a 7.35-m headline trawl, and a beam rigged directly to a 9.19-m trawl (3) with and (4) without a horizontal wire and plastic streamers. Despite more surface area (7.5 vs 6.0 m), both beam-trawl configurations had significantly lower drag than the otter trawls (≤30%). When catches were standardised to per ha, the otter trawl with sweeps retained significantly more (1.3-2.4 times) school prawns, Metapenaeus macleayi (Haswell), than the other three configurations. Within systems, removing sweeps or adding a horizontal wire significantly reduced the unwanted catches of a key teleost (southern herring, Herklotsichthys castelnaui Ogilby) by 41 and 48%. The results illustrate the utility of simple anterior modifications for independently addressing penaeid-trawling environmental issues.

Comparison of Tilt Variation in the Otter Board and Codend of Bottom Trawl Gear during Fishing Operations

The motion of the otter board on a trawl can affect the motion of trawl nets, and the motion of the codend can affect fish selectivity. Preliminary measurements of the tilt of bottom trawl gear were carried out to compare the tilts of the otter board and codend. The tilt of the otter board was measured by Vector and tilt at 1.5 m anterior to the end of the codend, and the middle upper panel was measured with a micro-DST-tilt logger. Tilt data such as yaw, pitch, and roll were analyzed by the fast Fourier transformation method and global wavelet and event analyses for the period or amplitude. The mean period <TEX>${\pm}$</TEX> standard deviation of the tilt in the otter board, <TEX>$(5-6){\pm}2s$</TEX>, was similar to the period of the codend, <TEX>$(4-6){\pm}(2-3)s$</TEX>, whereas the amplitude of the codend was greater than that of the otter board. The yaw and pitch periods were not significantly different between the otter board and codend, but roll was different. Furthermore, the tilt period frequencies of the otter board and codend were not significantly different. Accordingly, the tilt motion of the codend was mostly related to the tilt of the otter board.

Estimating historical trawling effort in the German Bight from 1924 to 1938

Based on historical landings and landings-per-unit-effort data, fishing effort was estimated in the period of 1924–1938 in a study area representing the German exclusive economic zone (EEZ) in the North Sea. Three main trawling fisheries were identified, i.e. the international flatfish fisheries comprising the effort of steam trawlers and motor cutters, the German shrimp fisheries and the industrial inshore fisheries. Except for shrimp trawling, which was performed by beam trawling with one beam per vessel, all historical trawling effort was identified as otter board trawling. The trawling effort increased from 1,131,000 to 1,232,000h in 1924 to 3,170,000 to 3,836,000h in 1937 and declined in 1938. The main fishery effort was contributed by the German shrimp fisheries, accounting for a maximum of 2,140,000h in 1937. In comparison, in 2006, approximately 540,000h were spent fishing in the trawl fisheries of the study area, comprising effort from small (≤221kW) to large beam trawlers (>221kW), shrimp fisheries and otter board trawling. The bulk of the historical effort was concentrated at water depths of 5–20m. With the implementation of combustion engines and new gear technology, the period of 1924–1938 marked a new era for marine fishing when virtually all space became accessible and, in particular, the trawling of inshore areas increased.

Straight Line Foraging in Yellow-Eyed Penguins: New Insights into Cascading Fisheries Effects and Orientation Capabilities of Marine Predators

Free-ranging marine predators rarely search for prey along straight lines because dynamic ocean processes usually require complex search strategies. If linear movement patterns occur they are usually associated with travelling events or migratory behaviour. However, recent fine scale tracking of flying seabirds has revealed straight-line movements while birds followed fishing vessels. Unlike flying seabirds, penguins are not known to target and follow fishing vessels. Yet yellow-eyed penguins from New Zealand often exhibit directed movement patterns while searching for prey at the seafloor, a behaviour that seems to contradict common movement ecology theories. While deploying GPS dive loggers on yellow-eyed penguins from the Otago Peninsula we found that the birds frequently followed straight lines for several kilometres with little horizontal deviation. In several cases individuals swam up and down the same line, while some of the lines were followed by more than one individual. Using a remote operated vehicle (ROV) we found a highly visible furrow on the seafloor most likely caused by an otter board of a demersal fish trawl, which ran in a straight line exactly matching the trajectory of a recent line identified from penguin tracks. We noted high abundances of benthic scavengers associated with fisheries-related bottom disturbance. While our data demonstrate the acute way-finding capabilities of benthic foraging yellow-eyed penguins, they also highlight how hidden cascading effects of coastal fisheries may alter behaviour and potentially even population dynamics of marine predators, an often overlooked fact in the examination of fisheries’ impacts.

Relative engineering and catching performances of paired penaeid-trawling systems

Two paired-trawl spreading mechanisms were compared, with a view towards progressing more environmentally benign penaeid-trawling systems. Identical twin trawls (7.35-m headline) were alternately towed in a “double rig” (four otter boards) with either 18- or 36-m bridles (“short-” and “long-” double rigs) and in a “dual rig” (36-m bridles with two otter boards and a sled). At similar speeds (∼1.31ms−1), wing-end spread per trawl was significantly greatest in the long-double (mean±SE: 4.50±0.05m), followed by the short-double (4.27±0.05m) and dual (3.86±0.05m) rigs. The dual rig required 24% and 20% less towing force and fuel and caught significantly fewer (by ∼60%) small mulloway, Argyrosomus japonicus per ha of foot-rope contact than both double rigs. Other standardised catches were not significantly different among configurations, although many, including school prawns, Metapenaeus macleayi were lower (by 25–39%) in the dual rig – which manifested as a relatively greater quantity of fuel required per kg caught (up to ∼1.5 times more). When school-prawn catches were combined across double rigs and compared against the dual rig, the latter caught significantly larger individuals, but fewer per ha of foot-rope contact (by 35%); an effect that was negated when catches were re-standardised to the contact of the foot-rope and otter boards. The results show that removing otter boards reduces benthic contact, towing force, fuel consumption and bycatch, but there are negative implications for penaeid catches that may require compensatory offsets.

Hydrodynamic characteristics of low aspect ratio vertical cambered otter board

Hydrodynamic characteristics of low aspect ratio vertical cambered otter board

Effects of otter boards on catches of an Australian penaeid trawl

The effects of otter boards on the catches of Metapenaeus macleayi were quantified within the broad aim of promoting more hydrodynamic and environmentally benign trawl-spreading mechanisms. Two identical trawls (7.26m footropes) were attached to 2.89-m sweeps and simultaneously fished in a double-rig; each alternately attached to either two otter boards or a rigid 6.0-m beam with parallel sleds (configured not to impact on the behaviour of M. macleayi). Both trawl configurations had the same total weight (143kg) and initial wing-end spreads (predicted means of 4.33 and 4.45m), although the otter boards caused significantly more variable wing-end spread during towing. Replacing the otter boards with the beam significantly reduced system drag (by 10%) and the weights and numbers of school prawns per hectare swept by the footropes (by 33%); with the latter attributed to the concomitant reduction in lateral bottom contact of the system. Standardised catches of the most abundant teleost Herklotsichthys castelnaui were also significantly lower (by 79%) in the beam trawl. The results support the potential for compensatory adjustments to penaeid-trawl parameters (e.g. footrope length) to offset reductions in the bottom contact of spreading mechanisms that are modified to minimise drag and/or habitat impacts.

An investigation of human vs. technology-induced variation in catchability for a selection of European fishing fleets

Abstract Mahévas, S., Vermard, Y., Hutton, T., Iriondo, A., Jadaud, A., Maravelias, C. D., Punzón, A., Sacchi, J., Tidd, A., Tsitsika, E., Marchal, P., Goascoz, N., Mortreux, S., and Roos, D. 2011. An investigation of human vs. technology-induced variation in catchability for a selection of European fishing fleets. – ICES Journal of Marine Science, 68: 2252–2263. The impact of the fishing effort exerted by a vessel on a population depends on catchability, which depends on population accessibility and fishing power. The work investigated whether the variation in fishing power could be the result of the technical characteristics of a vessel and/or its gear or whether it is a reflection of inter-vessel differences not accounted for by the technical attributes. These inter-vessel differences could be indicative of a skipper/crew experience effect. To improve understanding of the relationships, landings per unit effort (lpue) from logbooks and technical information on vessels and gears (collected during interviews) were used to identify variables that explained variations in fishing power. The analysis was undertaken by applying a combination of generalized additive models and generalized linear models to data from several European fleets. The study highlights the fact that taking into account information that is not routinely collected, e.g. length of headline, weight of otter boards, or type of groundrope, will significantly improve the modelled relationships between lpue and the variables that measure relative fishing power. The magnitude of the skipper/crew experience effect was weaker than the technical effect of the vessel and/or its gear.

The modeling of single-boat, mid-water trawl systems for fishing simulation

A mathematical model of single-boat, mid-water trawl system, together with a graphical interface have been created for training purposes. The model consists of separate components for the trawler and fishing gear. The trawler component considers the influence of wind and current and can be used to simulate trawlers in ordinary-speed and low-speed scenarios. The fishing gear component considers the towing warp, the otter board, and the fishing net. The model components are connected by coordinating boundary conditions. All trawling processes, including cruising, towing, and hauling and deploying the fishing net, can be simulated using this approach. Simulations have been undertaken based on a single-boat, mid-water trawler. The simulated trawler speed and turning circle under cruising conditions were compared to sea trials, and simulated trawl mouth height and trawl resistance to flume experiments. The hydrodynamic performance of a single-boat, mid-water trawl has been analyzed, showing that trends in fishing gear parameters together with towing speed and warp length are intuitively correct, but trawling speed is overestimated, which requires that the model component for the controllable pitch propeller be improved.

Etude dynamique d'un train de pêche complet

ABSTRACT We present a complete numerical model of a full trawl gear and the vessel. The trawler is modelled by equations and hydrodynamic coefficients which are classically used for ship behaviour and ship manoeuvrability studies. It is submitted to the action of the warps. The warps and the net are part of a complex fluid-structure coupling problem. The net is a supple structure and its shape depends on the forces that are acting on it. Now, these forces themselves depend of the net meshes shape. The otter boards are most particular wings. We have studied one particular type. Then all these elements have been put together by writing their dynamics equations. A time step method enables the calculation of the shape, the forces and the motion of each element in the whole system.

Fisheries in the context of marine spatial planning: Defining principal areas for fisheries in the German EEZ

A method is presented to define principal areas for fisheries at high spatial resolution applicable to be implemented into marine spatial planning procedures. Vessel monitoring system (VMS) data from 2005 to 2006 are acquired to determine vessel-based fishing effort. Principal areas for the German exclusive economic zone (EEZ) are defined as areas in which 75% of the effort of either year is carried out. Examples are given for the 5 most abundant fisheries in the German EEZ in terms of vessel-based effort, i.e. gill netting, pelagic trawling, demersal otter board trawling and beam trawling >300 and <300 HP. A historical comparison for demersal otter board trawling shows relative stability of spatial utilization patterns in the North Sea section of the EEZ.

평판형 전개판의 3차원 운동 모델링

Otter boards in the trawl are the one of essential equipments for the net mouth to be spread to the horizontal direction. Its performance should be considered in the light of the spreading force to the drag and the stability of towing in the water. Up to the present, studies of the otter boards have focused mainly on the drag and lift force, but not on the stability of otter boards movement in 3 dimensional space. In this study, the otter board is regarded as a rigid body, which has six degrees of freedom motion in three dimensional coordinate system. The forces acting on the otter boards are the underwater weight, the resistance of drag and spread forces and the tension on the warps and otter pendants. The equations of forces were derived and substituted into the governing equations of 6 degrees of freedom motion, then the second order of differential equations to the otter boards were established. For the stable numerical integration of this system, Backward Euler one of implicit methods was used. From the results of the numerical calculation, graphic simulation was carried out. The simulations were conducted for 3 types of otter boards having same area with different aspect ratio(<TEX>${\lambda}=0.5,\;1.0,\;1.5$</TEX>). The tested gear was mid-water trawl and the towing speed was 4k't. The length of warp was 350m and all conditions were same to each otter board. The results of this study are like this; First, the otter boards of <TEX>${\lambda}=1.0$</TEX> showed the longest spread distance, and the <TEX>${\lambda}=0.5$</TEX> showed the shorted spread distance. Second, the otter boards of <TEX>${\lambda}=1.0$</TEX> and 1.5 showed the upright at the towing speed of 4k't, but the one of <TEX>${\lambda}=0.5$</TEX> heeled outside. Third, the yawing angles of three otter boards were similar after 100 seconds with the small oscillation. Fourth, it was revealed that the net height and width are affected by the characteristics of otter boards such as the lift coefficient.